Heart failure (HF) is a debilitating global disease whose incidence is increasing dramatically. Heart failure refers to a clinical syndrome in which an abnormality of cardiac function causes a below normal cardiac output that can fall below a level adequate to meet the metabolic demand of peripheral tissues. Heart failure can be due to a variety of etiologies with ischemic heart disease being the most common. Heart failure can be treated with a drug regimen designed to augment cardiac function or by pacing therapy. It has been shown that some heart failure patients suffer from intraventricular and/or interventricular conduction defects (e.g., bundle branch blocks) such that their cardiac outputs can be increased by improving the synchronization of ventricular contractions with electrical stimulation. In order to treat these problems, implantable cardiac devices have been developed that provide appropriately timed electrical stimulation to one or more heart chambers in an attempt to improve the coordination of atrial and/or ventricular contractions, termed cardiac resynchronization therapy (CRT). Ventricular resynchronization is useful in treating heart failure because, although not directly inotropic, resynchronization can result in a more coordinated contraction of the ventricles with improved pumping efficiency and increased cardiac output. Currently, a most common form of CRT applies stimulation pulses to both ventricles, either simultaneously or separated by a specified biventricular offset interval, and after a specified atrio-ventricular delay interval with respect to the detection of an intrinsic atrial contraction or delivery of an atrial pace.
The treatments for HF discussed above are most appropriately used to treat patients with systolic dysfunction. In such patients, the fraction of the blood accumulated during diastolic filling of one or both ventricles that is pumped out during systole, referred to as the ejection fraction, is below normal. Recent epidemiological reports, however, have documented that a large percentage of HF patients maintain a normal left ventricular ejection fraction (EF)≧50%. Although current treatment modalities have improved prognosis among individuals with a depressed EF, the application of these therapeutic strategies to the condition of HF with a normal EF have not had such favorable outcomes